This invention relates to methods and apparatus for the production and distribution of a carbon dioxide slurry, and more particularly to methods and apparatus for transporting such a slurry from a production site to a remote customer station for utilization thereat.
In numerous applications, such as for example food freezing processes, it has been found economical to obtain necessary refrigeration from consumable refrigerant products. For example, liquid carbon dioxide which is commonly supplied at 300 p.s.i.g. and 0.degree.F, is in widespread use as such a refrigerant product. Solid carbon dioxide, commonly known as "dry ice" is also used as a refrigerant in certain applications, Although solid carbon dioxide yields, 1.9 times as much refrigerant capability (B.T.U.) per pound of material as compared with liquid carbon dioxide, use of solid CO.sub.2 does not find widespread acceptance due to the difficulty in handling this product. As liquid carbon dioxide is more susceptible to simpler handling and transport techniques, inasmuch as liquid CO.sub.2 may be readily pumped, utilization of liquid CO.sub.2 for refrigeration purposes is preferred over use of this material in solid form for large scale or tonnage applications.
It has been found that by maintaining carbon dioxide at substantially triple point conditions, i.e. 60 p.s.i.g., -70.degree.F, a slurry comprised of finely divided, unagglomerated particles of solid CO.sub.2 suspended in liquid carbon dioxide may be produced. It has also been found that such a carbon dioxide slurry contains greater refrigeration capacities than an equivalent weight of liquid carbon dioxide. In addition, it has been observed that the greater the solids content of a CO.sub.2 slurry, the greater is the refrigeration capacity of each unit of slurry product. However, as the solids concentration of a slurry increases, the difficulties of pumping the slurry and the likelihood of clogging slurry conduits increase accordingly. Nonetheless, the economies of utilizing carbon dioxide slurry as a refrigerant product may be exploited upon the careful handling thereof.
While a carbon dioxide slurry may be produced in a reaction vessel, as will be described in detail hereafter, certain difficulties have been encountered in the subsequent transportation and distribution of such a slurry. It has been found that once solid carbon dioxide particles, which preferably exhibit a maximum cross-sectional dimension of 4 microns to 2 millimeters, are suspended in liquid carbon dioxide in slurry form, particle size cannot be reduced during subsequent handling operations. Therefore, unless particles of solid CO.sub.2 are formed with approximately the foregoing dimensions, handling apparatus, such as conduits and pumps, are readily plugged or clogged. In addition, it has been observed that upon standing, solid carbon dioxide particles tend to settle to the bottom of a container vessel and although such particles do not agglomerate, settled particles must be reslurried before slurry product can be discharged from the vessel. Accordingly, the transport apparatus, such as a slurry trailer, must either provide an agitating device or further pump means to reslurry settled solid particles prior to discharge of the slurry. Retention of the slurry in a suitable storage vessel at a remote or customer location also requires that some means, i.e., an agitator device, pump, etc., be provided for maintaining the slurry in a pumpable condition with solid particles suspended in liquid carbon dioxide. Furthermore, in order to preserve the economical advantages of a carbon dioxide slurry, the capital and operating costs incurred in providing apparatus for maintaining a suspension of solid CO.sub.2 particles in liquid carbon dioxide must be minimized.